1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, a reversible thermosensitive recording label, a reversible thermosensitive recording device, an image processing apparatus, and an image processing method.
2. Discussion of the Background
Thermosensitive recording media utilizing coloring reaction between an electron donating coloring compound (hereinafter referred to as coloring agent) and an electron accepting compound (hereinafter referred to as coloring developer) are well known. With development of office automation, these media are widely used in outputs for computers, facsimile machines, automatic ticket machines, printers for scientific measuring equipment, printers for medical measuring equipment, and magnetic thermal cards such as prepaid cards and reward cards.
However, these thermosensitive recording media are irreversible so that once-used media are inevitably discarded, which leads to an environmental problem. Therefore, in light of recycling use, various kinds of thermal recording media utilizing the coloring reaction between a coloring agent and a coloring developer have been proposed. For example, published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) S60-193691 describes a thermosensitive medium using the combination of gallic acid and fluoroglucinol as a coloring agent. In this application, the coloring compound obtained through thermal colorization is decolored by using water or vapor. However, this medium has a drawback in improvement on water resistance and record preservation property, resulting in size increase in a decolorization device to decolor the coloring compound. JOP S61-237684 describes a rewritable recording medium in which a compound of phenolphthalene, tymolphthalene, bisphenol, etc. isused as a coloring developer. This application describes that a coloring compound can be formed by heating and gradually cooling down the recording medium, and the coloring compound can be decolored by heating the medium to a temperature higher than the coloring temperature and cooling down the medium. However, in addition to the complexity of the processes of colorization and decolorization, the recording medium of this application has a drawback in that the decolorized state achieved after decolorization of the coloring compound still shows color in some degree and therefore it is not possible to obtain an image having a good contrast.
In addition, JOPs S62-140881, S62-138568, and S62-138556 describe a color memory printing material containing a uniform compatible compound formed of a coloring agent, a coloring developer and a carboxylic acid. In these applications, the material achieves a complete colored state at a low temperature, and a complete decolored state at a high temperature. Further, the material can maintain a colored or decolored state at an intermediate temperature. Therefore, it is possible to record white characters (decolored state) on a colored background (colored state) by using a thermal head. However, since the recorded image is a negative image, its usability is limited, and further it is necessary to maintain a recorded image in a particular temperature range to preserve the recorded image.
JOPs H2-188294 and H2-188293 describe a reversible thermosensitive recording medium using a salt of gallic acid and a higher fatty acid as a coloring developer by which coloring and subtractive coloring are reversibly performed. These media can thermally color the salt in a particular temperature range and decolor the salt at a higher temperature than the range. However, since the coloring and subtractive coloring are performed in a competitive manner, there is a drawback in that it is difficult to thermally control these coloring and subtractive coloring reactions and resultantly hard to obtain an image with good contrast.
As mentioned above, conventional reversible thermosensitive recording media using the reaction between a coloring agent and a coloring developer involve various kinds of drawbacks and are not satisfactory in light of practical use. In addition, these media are totally insufficient as a reversible multi-color thermosensitive recording medium.
Therefore, the inventors of the present invention describe in JOP H5-124360 a reversible thermocoloring material containing an electron donating coloring compound and an electron accepting compound which achieves a colored state by temporarily heating the material to a temperature not lower than its melting point, and a decolored state by temporarily heating the material to separate and crystallize electron accepting coloring compound. The reversible thermocoloring material utilizing the reaction between the coloring agent and the coloring developer described in this application can resolve the drawbacks mentioned above and thermally achieve a colored state and a decolored state with ease. Further, the colored state and the decolored state are stably maintained at room temperature. However, the reversible thermocoloring material is not stable to light. Therefore, the material has a drawback in that, when the recording surface in a colored state in the medium is irradiated with light, its background may be discolored or the density of a decolored state does not lower to the same level as that of the background, resulting in insufficiency of decolorization.
Consequently, a reversible thermosensitive recording medium which has a good light resistance, and repetitively achieves a stable colored state and decolored state in which decolorization is almost complete even when exposed to light, has not been provided.